1. Field of the Invention
This invention pertains generally to tomography, and more particularly to a methodology for using an object's surroundings as a parameter in image reconstruction for the purposes of dose reduction and image enhancement.
2. Description of Related Art
Tomographic imaging systems, such as medical X-ray CT or CAT scanners, produce 2D or 3D images of the internal structures of an object through the mathematical reconstruction of projections taken from many different directions or angles. The typical tomography apparatus, for example, contains a radiation source and a detector that is rotated around an axis extending perpendicularly from the plane of the examination table. Projections of the patient or object are conventionally taken at equal angle intervals such that the angle of the radiating source with respect to the isocenter of the scanner changes by a fixed degree from one projection to the next. The projections are mathematically reconstructed by computers using one of many reconstruction algorithms that have been developed to produce 2D or 3D cross-sectional images of the subject of interest. These algorithms strike a balance between accuracy and the use of computational time and resources. Images have been produced from several different beam sources including x-rays and photons of other energies, electrons, protons, neutrons, sound waves and others.
The image quality of a 2D or 3D image reconstruction from projectional data is strongly dependent on the number of projections that are available. Low contrast structures and the general geometrical fidelity of the reconstruction are lost if the number of projections is below a certain level, which is traditionally quantified by the Nyquist criteria and the Shanon sampling theorem. However, the number of projections that can be obtained is limited by such factors as radiation dose to the patient, practical limitations of the imaging system, and temporal constraints. A critical problem encountered in tomographic imaging is the radiation dose that is imparted to the patient or biological specimen as the result of the imaging procedure. Since the radiation dose is proportional to the number of projections that are taken, and since by its nature tomographic imaging requires a high number of projections for a suitable reconstruction, common procedures such x-ray CT impart a significant radiation dose to patients results in a potential for carcinogenesis as a result of the imaging procedure. Furthermore, with the increasing popularity of medical x-ray CT and fluoroscopic imaging procedures, the long term effects of exposing patients to such ionizing radiation has become an increasing concern, especially for pediatric patients.
The image reconstruction process is analogous to solving a linear system of equations with the unknowns being the voxel values and the equations being the projections through the patient at different positions and angles. The higher the number of unknown voxels, the higher the number of projections that are needed to produce an image of a suitable quality. Conventional tomographic imaging systems in clinical use today use computer programs that typically solve for the entire set of voxel values in the whole volume of the field of view of the scanner including regions of the object's surroundings such as the bed, surrounding air, and surrounding modules.
Current commercial clinical reconstruction algorithms do not use the patient's/object's surroundings in the reconstruction process as it is not obvious how to determine such information about the surroundings for each specific scan setting or how to use this information in the reconstruction process. Furthermore, there exists considerable variation in patient size and shape and positioning of scanner modules as a function of slice location and scan parameters. As a result of reconstructing the full field of view and not utilizing the surrounding information, conventional methods require a high number of projections to reconstruct an object to a satisfactory level of quality. Since the number of projections is related to the radiation dose to the object, such methods lead to higher patient/object doses than is desired.
There is a need for a system and method for tomographic imaging that limits the exposure of the subject to potentially harmful or destructive radiation that is at the same time accurate, reliable and computationally practical. The present methods satisfy these needs, as well as others, and are generally an improvement over the art.